The present invention relates to receivers for communication systems, and particularly relates to radio frequency receivers that include a signal filter for isolating a communication signal.
Conventional communication systems typically include a transmitter, a communication channel and a receiver. Many communication channels generally operate more efficiently when a relatively high frequency communication signal is being transmitted. Receivers in such communication systems, therefore, typically down-convert the received signal to a relatively lower frequency for signal processing. Most such receivers also include a filter for removing system noise and/or signals other than a particular communication signal of interest. In many systems, the received filtered communication signal of interest is relatively weak, and most such receivers, therefore, further include an amplifier for amplifying the received filtered communication signal of interest.
A conventional communication signal receiver includes a high-gain channel selection filter that provides selective filtering as well as high gain amplification. A difficulty with such receivers, however, is that the circuit offset may merge with the signals or saturate the circuit after significant amplification. Although one approach to correcting for this is to provide AC coupling of the filter output, for example by connecting an offset-free high-pass filter in cascade with a conventional filter, there are disadvantages to such an approach. First, the offset will remain inside the filter itself, leading to loss of dynamic range or even saturation. Second, since the signal will be required to travel through the high-pass filler in addition to the other components, the signal may be further degraded or distorted, e.g., the signal-to-noise ratio and distortion may be compromised.
Another approach involves employing a low pass filter in a feedback path that provides inverted offset to the input of the channel selection filter. For example, as shown in FIG. 1, such a receiver 10 includes a combiner 12 that combines an input signal with a feedback signal and produces a combined signal that is input to a first low pass filter 14. The output of the first low pass filter 14 is fed back to the combiner 12 after passing through a second low pass filter (e.g., an integrator) 16, and may also be provided to an analog to digital converter 18 for processing. The use of a first low pass filter 14 in the forward path and a second low pass filter 16 in the feedback path provides the equivalent of a bandpass filter. In order to achieve the low frequency high-pass corner on a very high gain channel selection filter, however, the size of components in the feedback path is large. In some cases, this requires that some components of the feedback path not be included on the chip itself, and instead be located off-chip, leading to reduced attractiveness of the chip. Moreover adding an offset correction loop may also contribute to additional noise in the receiver signal.
There is a need, therefore, for a receiver circuit that may achieve the above objectives without significant disadvantages, such as large component sizes and noise addition.